In the field of microelectric devices, optics, sensors and biochemical sensors, among others, such devices require patterning of materials, which may be conductive, semi-conductive or dielectric. Traditional techniques utilize photolithographic processes to form patterns. However, such traditional techniques require extensive processing, relatively sophisticated equipment/materials, and is time-consuming. For example, according to photolithography techniques, a negative or positive photoresist is coated onto a thin film of conductive, semi-conductive or insulating material on a substrate. The photoresist is then irradiated in a desired pattern and portions of the resist (in some cases, the irradiated portions and, in other cases, the non-irradiated portions) are washed away. To form a pattern of conductive, semi-conductive or insulating material, such material that is not covered by the remaining photoresist is then removed, followed by removal of the remaining photoresist. What remains on the substrate is a pattern of the conductive, semi-conductive or insulating material. As described above, such photolithographic techniques require extensive processing and take a significant amount of time. Further, developing small features (e.g., less than 100 nm) and patterning on non-planar substrates still pose challenges to such photolithographic technique.
Several non-lithographic techniques have been demonstrated for fabricating microstructures. Soft lithography techniques, for example, are used to prepare microstructures, among them, micro contact printing (μCP), replica molding (REM), embossing, etc. Contact printing is a flexible, non-lithographic method for forming patterned materials.
Microcontact printing, for example, allows patterns of microparticles to be imparted onto a substrate surface.
U.S. Pat. No. 6,180,239 to Whitesides et al. discloses a method of forming a patterned self-assembled monolayer on a surface and derivative articles are provided. According to one method, an elastomeric stamp is deformed during and/or prior to using the stamp to print a self-assembled molecular monolayer on a surface. According to another method, during monolayer printing the surface is contacted with a liquid that is immiscible with the molecular monolayer-forming species to effect controlled reactive spreading of the monolayer on the surface. Methods of printing self-assembled molecular monolayers on nonplanar surfaces and derivative articles are provided, as are methods of etching surfaces patterned with self-assembled monolayers, including methods of etching silicon. Optical elements including flexible diffraction gratings, mirrors, and lenses are provided, as are methods for forming optical devices and other articles using lithographic molding. A method for controlling the shape of a liquid on the surface of an article is provided, involving applying the liquid to a self-assembled monolayer on the surface, and controlling the electrical potential of the surface.
U.S. Pat. No. 7,338,613 to Schueller et al. discloses an automated process for microcontact printing is provided, comprising the steps of providing a substrate and a stamp; automatically aligning the substrate and stamp so that the stamp is aligned relative to the substrate to impart a pattern to the substrate at a desired location and with a desired orientation on the substrate; applying an ink to the stamp, the ink including a molecular species adapted to form a self-assembling monolayer (SAM) on the substrate; contacting the stamp and the substrate; and separating the stamp from the substrate.
Microcontact printing techniques are also less costly and time-consuming than traditional photolithography processes since it is procedurally less complex, ultimately not requiring spin coating equipment or a sequential development step. These techniques use an elastomeric stamp to transfer the pattern and to form SAMs patterns while transferring molecules of the “ink” to the surface of the substrate by contact. Usually, alkanethiolates SAMs are formed on gold and can be widely used in microelectronics. Patterned inks were formed essentially on planar surfaces, very few trials were reported on non-planar substrates where μCP was used to form pattern of gold on a gold coated glass capillaries with hexanedecanethiol followed by selective etching in an aqueous solution of cyanide. SAM printing is capable of creating high resolution patterns, but is generally limited to forming metal patterns of gold or silver with thiol chemistry. Thiol chemistry is normally associated with a dispersing agent.
In SAM printing, a positive relief pattern provided on an elastomeric stamp is inked onto a substrate. The relief pattern of the elastomeric stamp, which is typically made of polydimethylsiloxane (PDMS), is inked with SAM molecules, e.g., thiol materials. This is traditionally necessary as without a dispersing agent, such thiol materials tend to undesirably agglomerate. The substrate is coated with a thin metal film of gold or silver. The gold or silver-coated substrate is then contacted with the stamp where a monolayer of the thiol material having the desired microcircuit pattern (i.e., of the relief pattern) is transferred to the metal film. Then when the substrate is then etched in, for example, a batch etching process, the SAM acts as an etch resist. The SAM un-protected metal areas are then etched away to the underlying substrate. The SAM is then stripped away leaving the metal in the desired pattern.
The drawbacks however are that microcontact printing via thiol chemistry is limited to only a few metals with resolutions on or the order of only 50 microns or less. Other drawbacks are the material to be printed does not effectively wet the surface of the stamp thus making for an incomplete pattern of material on the substrate.